1. I don’t add it to APA or AIPA. If you’re using all extract, you could add some sugar to a recipe to ensure you get a low enough FG, since extracts are not as fermentable. I wouldn’t use over ~ 10%.
2. It would prevent excessive darkening of your wort, which is another drawback of using a high % of extract.
Lower fg will give you a drier beer. With some extracts, especially darker, your beer can end at a higher fg than you want and be sweeter than you desired or have more body or a heavier mouthfeel.
I use the lightest dme I can find (breiss pils) and add it late. I have no issues with darkening or high fg. Sugar will help to boost your abv without adding body. Belgian triples are a good example. If your going all extract on an IPA sugar might help.
Adding extract late in the boil also improves hop utilization. When I brewed partial boil with extract I would normally add roughly half at the start and the rest towards the end. With full boil extract, it is less of an issue in both the color and hop utilization front.
I’ve added sugar to every extract beer I’ve done for the last 10 years. It was the key to making the rye extract from NB produce a rye IPA very much like the AG version.
I’ve been adding the majority of malt extract during last 10 minutes of the boil. Since doing this, I’ve noticed less caramelization which keeps the wort lighter color. I’ve also noticed better hop utilization, which is very important to me because I’m also doing partial boils (4-4.5 gal) and topping-off, and that already affects hop utilization.
I just tried adding simple sugar (dextrose in my case) — only 5% of the fermentables, and added it during the last 5 minutes. I’ve read that it help dry it out, and lo and behold — it dropped my FG down a point as per the target. It was a west-coast style IPA, but it’s still in the carboy, and the sample I took for a reading did taste somewhat drier than I had expected.
If I add 5% sugar to a recipe, would I just find what 5% of my extract weight is? For example, 5 lbs of DME, I would use .25 lbs of sugar and 4.75 lbs DME?
I would count them. Steeping grains may not necessarily contribute fermentables, but in the case of something like crystal malt they will definitely contribute sweetness and % of total grain bill is a good benchmark to track how much crystal you are adding.